The present invention relates to charge-coupled devices and in particular to a charge-coupled device exhibiting a tilted channel potential in each cell thereof.
A charge-coupled device (CCD) includes an array of closely spaced cells aligned along a lateral path. Each cell includes an electrode formed on an oxide layer covering a semiconductor substrate with the substrate of each cell having a channel region for storing charge carriers. When electrodes of neighboring cells are biased by clock signals of differing phase, an electric field develops between channel regions within or on the surface of the substrate beneath the electrodes of the cells and this electric field drives charge carriers stored in the channel region of one cell into the channel region of its neighboring cell. By applying appropriately phased clock signals to electrodes of neighboring CCD cells, charges are shifted laterally from cell-to-cell.
During initial stages of charge transfer between adjacent CCD cells, the clock-induced potential gradient between the channel regions of the cells provides a strong electric field driving carriers quickly into the receiving cell. A high carrier concentration gradient between channel regions of the adjacent cells also encourages carrier flow by diffusion. However as charge carriers begin to accumulate in the receiving cell, both the carrier concentration gradient and the potential gradient between the channel regions of the adjacent cells decrease, thereby slowing diffusion and drift of remaining carriers into the channel region of the receiving cell. As the frequency of clock signals applied to the electrodes of CCD cells increases, the time available for all charge carriers to move from one cell to its neighboring cell decreases. At high clock frequencies, a substantial portion of the charge remains behind at the end of a transfer cycle.
The "charge transfer efficiency" of a charge-coupled device is the ratio of charge transferred to a cell from its neighboring cell during a clock phase to the initial charge stored under its neighbor electrode at the beginning of the clock phase. As the frequency of operation of a charge-coupled device increases, charge transfer efficiency decreases. A high charge transfer efficiency is desirable, particularly in large charge-coupled devices, to prevent substantial degradation of charges passing through the charge-coupled device. Charge transfer efficiency of a charge-coupled device can limit its frequency of operation.